Optimize string-like portable Span APIs for Ordinal and OrdinalIgnoreCase

[ahsonkhan]

This is specific to portable span (and not fast span).

We do not need to allocate and call the string APIs when StringComparison is Ordinal or OrdinalIgnoreCase.

Similar to APIs like EndsWith/etc., add a non-allocating code path for CompareTo for Ordinal:
https://github.com/dotnet/corefx/blob/42d5370f8c6cec8d1961f54a0974e52b0562b0a7/src/System.Memory/src/System/MemoryExtensions.Portable.cs#L23

Also optimize for OrdinalIgnoreCase for all the string-like APIs.

See: dotnet/corefx#27319 (comment)

cc @tarekgh

[benaadams]

There are Ordinal versions in mscorlib/shared/System/StringSpanHelpers.cs ?
e.g. CompareOrdinalIgnoreCase(ReadOnlySpan<char>,ReadOnlySpan<char>) was looking at them in dotnet/coreclr#16434

[ahsonkhan]

There are Ordinal versions in mscorlib/shared/System/StringSpanHelpers.cs ?

Sorry, this issue is specific to portable span only. Fast span already leverages that API.